The present invention relates to a projector equipped with a DMD (digital micro-mirror device).
A projector is provided where an image is generated by an image display device receiving an image data from a personal computer or a video camera and projected on a screen. The projector hence includes an optical system for projection of images.
Such an optical image projection system has a known construction shown in FIG. 6. A beam of light emitted from a lamp 116 is passed through a lens 151, rendered coloring by a color wheel 119, passed through a lens 152, and projected on a DMD 113. As the light is reflected on an image generating surface of the DMD 113, it generates an optical image which is then magnified by the action of a projection lens 105 to generate a visible image 154 on a screen 153.
For reducing its size, the optical image projection system may have an optical device known as a light tunnel or a reflecting mirror provided on the light path. FIG. 7 illustrates an example of such a type of the optical image projection system where a beam of light emitted from a lamp 116 is received by a filter 118 to remove its ultraviolet and infrared components. The light is then rendered coloring by a color wheel 119 and passed through a light tunnel 120. The light tunnel 120 allows the light received at one end to be repeatedly reflected in a short optical path before released from the other end, and generates a uniform light. The light tunnel 120 hence contributes to the reduction of the light path length, and makes the optical image projection system compact in the dimensions. The light through the light tunnel 120 is then corrected in the wavelength by a tablet lens 115 and reflected on a reflecting mirror 114. The reflecting mirror 114 allows the light from the lamp 116 to be reflected and directed towards the DMD 112 as its light path is bent. This causes the optical image projection system to be more decreased in the overall size. The light reflected by the reflecting mirror 114 is then converged on the DMD 112 by the action of a relay lens 161. The converged area is confined to the mirror surface of the DMD 112 by the action of a black aluminum stop 113 (referred to as a DMD stop hereinafter).
The reason why the convergence of the light has to be confined to the mirror surface of the DMD 112 by the DMD stop 113 is explained below. In common, an area wider than the mirror surface of the DMD 112 is irradiated by the light from the relay lens 161 towards the DMD 112. If the convergence of the light is not confined by the DMD stop 113, a reflected light from other surfaces than the mirror surface may enter the projection lens 105 and interrupt the projected image. Also, the DMD 112 may entirely be heated up by the converged light.
One of the prior arts is disclosed in Japanese Patent Publication No. 2000-258703 (referred to as a first prior art hereinafter). Its projector is designed for generating a projected image at high contrast with no ghost effect, where a prism is installed between the DMD and the projection lens. The prism passes a light directed towards the projection lens by a DMD mirror unit, but reflects a light directed towards other than the projection lens by the DMD mirror unit. Thus the light reflected towards other than the projection lens by the DMD mirror unit is inhibited from entering the projection lens.
Another prior art is disclosed in Japanese Patent Publication No. 2002-107822 (referred to as a second prior art hereinafter). Its projector is arranged with its projection lens having both curved surfaces modified such that the DMD entering light fluxes of which the incident angle is substantially equal in the absolute value to but different in the sign from the normal light can be held in a not-converging state on the screen, hence preventing the light from the DMD from reflecting on the surface of the projection lens and entering again the DMD to generate a ghost effect in the projected image.
A further prior art is disclosed in Japanese Patent Publication No. 2000-206452 (referred to as a third prior art hereinafter). Its projector is designed for narrowing the flux of light at the entrance pupil of the projection lens to improve the luminance of the projected image on the screen with using no complex lens system. In the projector a condenser lens is installed between the color prism and the DMD.
A still further prior art is disclosed in Japanese Patent Laid-open Publications 2000-321529 (referred to as a fourth prior art hereinafter). Its projector is arranged with a plurality of small-power light sources for illumination on the DMD at higher efficiency. In the projector, rays of light from the plural light sources are combined by a fly-eye lens to a uniform luminance light.
However, the convention projector with the above described optical projection system shown in FIG. 7 has a drawback that as the intensity of light directed from the lamp 116 via the reflecting mirror 114 and the relay lens 161 to the DMD 112 is significantly high, a light reflected towards other than a projection lens 105 by a DMD mirror unit 131 shown in FIG. 8 reaches as a dispersed light the projection lens 105 regardless of the use of the DMD stop 113. Also, the light reflected towards other than the projection lens 105 by the DMD mirror unit 131 may heat up the DMD stop 113 and cause its thermal fracture. Moreover, while the DMD stop 113 is made of an aluminum sheet and has its surfaces anodized having a black color, its opening 113b shown in FIG. 8 is provided by cutting out the anodized aluminum sheet, thus causing the edge side 113a at the opening 113b to be exposed as of not anodized aluminum. This permits the light reflected towards other than the projection lens 105 by the DMD mirror unit 131 and the reflected light 142 from other surfaces than the DMD mirror unit 131 to be mirror reflected on the exposed edge side 113a and then reach the projection lens 105 thus declining the contrast in the projected image.
The first to fourth prior arts also fail to eliminate the foregoing drawback.